Patent document 1 (for example, Japanese Patent No. 3863031) discloses a technique for monitoring a state of a battery, which is constituted of plural secondary cells connected to each other in series in order to boost output voltage thereof, when the battery is being charged and discharged. Patent document 1 discloses a technique which prevents an occurrence of a mode in that voltage of the battery can not be supplied to a load by cancelling a charge inhibit signal, in a case where the charge inhibit signal is input to a microcomputer when overcurrent is detected.
Patent document 2 (for example, Japanese Patent Laid-Open Publication No. 2008-278688) discloses a protection circuit which keeps cell balance when battery voltage becomes greater than a predetermined voltage. According to this technique, it is possible to keep the cell balance when the battery voltage becomes greater than the predetermined voltage.
Herein, a technique for keeping the cell balance is a technique in that variability of amount of charge of each cell included in a battery is corrected. The variability of amount of charge is caused by variability of discharge rate of each cell. It is necessary to correct the cell balance particularly for a large battery in which a number of cells are connected to each other in series.
According to the technique disclosed in patent document 2, each cell includes an over discharge detecting circuit which detects over discharge of the cell, an overcharge detecting circuit which detects overcharge of the cell and a controller which controls turning on/off of a switch that is connected to the cell in parallel. In Order to keep the cell balance, the controller turns on the switch based on a detection result of the overcharge detecting circuit when the battery is being charged. When the battery is being discharged, the overcharge detecting circuit and the controller are turned off based on detection result of the over discharge detecting circuit in order to keep the cell balance by decreasing power consumption and thereby decreasing discharge rate of the cell.
In a case where battery voltages of the cells include variability, and then battery voltage of one of the cells decreases and becomes less than over discharge detect voltage faster than battery voltages of other cells, a state monitoring circuit of the battery detects decrease of battery voltage of the cell and discharge of the cell is suspended based on detection result of the state monitoring circuit of the battery. In this condition, power consumption of the cell becomes lower than other cells, and thus discharge rate of the cell becomes slower than other cells. Since other cells are discharged normally, it becomes possible to keep the cell balance between the cell and other cells.
On the contrary, in a case where battery voltages of the cells include variability, and then battery voltage of one of the cells increases and becomes greater than predetermined cell balance voltage faster than battery voltages of other cells, the switch which is connected in parallel to the cell is turned on. Then the cell is discharged, and it becomes possible to keep the cell balance.
According to the technique as described above, each cell includes the over discharge detecting circuit which detects over discharge, the overcharge detecting circuit which detects overcharge and the controller which controls turning on/off of the switch that is connected in parallel to the cell. In order to keep the cell balance, the controller turns on the switch based on the detection result of the overcharge detecting circuit when the battery is being charged. When the battery is being discharged, the overcharge detecting circuit and the controller are turned off based on detection result of the over discharge detecting circuit in order to keep the cell balance by decreasing power consumption and thereby decreasing discharge rate of the cell.
When over discharge of a cell is detected, the controller does not control turning on/off of the switch, and the overcharge detecting circuit and the controller are turned off. Then the power consumption is reduced and the discharge rate of the cell is decreased in order to keep the cell balance. In this condition, it is not possible to increase discharge rate of other cells in a positive manner.
Since the switches are controlled to be turned on/off independently for each cell, the problem described above is caused.
Since the switches are controlled to be turned on/off independently for each cell, it is not possible to keep the cell balance in a manner that all of the switches are turned on/off relative to one another, when the cells are discharged and charged.
According to the technique described above, the cell balance can be kept by switching the switches only when the battery is being charged, since the switches are controlled to be turned on/off independently for each cell. Thus, there is a problem in that it is not possible to keep the cell balance by switching the switches when the battery is being discharged. There is another problem in that it is not possible to keep the cell balance in a manner that all of the switches are turned on/off relative to one another, when the cells are discharged and charged.